In cabinets, showcases, vending machines, freezers and refrigerators, panels, sinks, switchboards, etc., metal sheets are used having edges thereof folded in cross sections of the shape of a channel or of the shape of the letter Z. The operation of forming such folds in a metal sheet generally involves first folding the shorter edges of a work cut in prescribed size and shape from a master metal sheet, rotating the work in its original plane at right angles to the shorter edges, and folding the longer edges of the work. In this case, when the longer edges are to be folded by a wing die, for example, since the work already contains the stand parts formed by folding along the shorter edges, the die for clamping the work, particularly the upper die, is required to have a length twice the thickness of the work less than the length of the longer edges of the work. It is, therefore, necessary that the die should be furnished with a multiplicity of upper dies of varying widths to permit interchange of such upper dies whenever there is a change of work width. The die of this principle inevitably entails expenditure of time and labor in the interchange of upper dies. Besides, this operation in most cases is performed manually and consequently prevents folding machines from being automated or adapted for an NC or CNC system. The desirability of a split die suitable for automatic control and an apparatus for interchange of segments of the die has found recognition.